Electric door strikes are commonly used in various places of business where it is desired to control entry into a secured area by means of a remote switch. As an example, the lobby of a building might be separated from the rest of the facility by a door that is secured by an electric door strike. When an individual or group of individuals has been cleared for entry into the main part of the building, the receptionist or security guard depresses a momentary switch causing the door strike to be unlocked for a set period of time. The door strike then returns automatically to the locked condition.
There are four general versions or operating modes of the electric door strike, commonly referenced as follows:
Fail Secure/Direct Control PA1 Fail Secure/Timed Release PA1 Fail Safe/Direct Control PA1 Fail Safe/Timed Release
In the Fail Secure mode, a loss of power leaves the door strike in the locked condition. The solenoid that drives the strike may be powered only briefly to unlock the door, and, because of the low duty cycle, the average power demand is low. In certain applications (i.e. employer entrance doors) the fail secure strike may be powered for eight or more hours. Reducing the voltage to a "hold-in level" after initial pull in is useful here also.
In the Fail Safe mode, the door strike is unlocked by a loss of power. Fail Safe strikes are powered continuously except while the door is unlocked. This constitutes a high duty cycle with relatively high average power demands. To prevent overheating in this operating mode, the voltage supplied to the solenoid should be reduced to a holding voltage after pull-in.
Under Direct Control, the strike is held in an unlocked condition as long as the switch is depressed. When the switch is released, the strike returns to the locked condition. Depressing the switch removes power to unlock for Fail Safe and applies power to unlock for Fail Secure.
In the Fail Secure/Timed Release mode or the Fail Safe/Timed Release mode, the doorstrike remains unlocked for a set period of time following a momentary switch closure. Again, power is removed to unlock under Fail Secure and is applied to unlock under Fail Safe.
The installers of electric door strikes are constantly confronted by a number of complications that arise because of the variety of operating modes. In addition, there is a lack of standardization in the industry relative to supply voltage for strike operation. Some strikes are designed to operate at 12, or 24 or up to 40 volts dc; others are designed for 12, 16 or 24 volts ac. Supply voltage from 12 to 40 volts dc or 12 to 28 volts ac may be present at a particular location, and the installer needs to match the device to the available voltage. One customer may require a relatively short release time; another may want a considerably longer release time. Because of the limited versatility and adjustability of prior art and presently available electric door strikes, the installer is required to stock a supply of the various versions of door strikes, and in some cases complicated adjustments have to be made at the site.
The goal of the present invention is to provide a versatile power and control circuit for an electric door strike that is immediately operable from any of the aforementioned voltage sources and is readily adaptable at the site for operation in any of the four different modes with readily adjustable release times. In addition, the present invention is directed toward the provision of a highly efficient power supply that permits miniaturization and prevents overheating in the confined space that is available for installation in the door jamb. The unit also minimizes heating of the door strike by reducing operating voltage to a hold-in level after initial powering.